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The study of the geology and ground-water resources of the lower South Platte River valley was made by the Ground Water Branch of the U. S. Geological Survey at the request of the U. S. Bureau of Reclamation and with the endorsement of the Colorado Water Conservation Board. The area includes parts of Colorado and Nebraska, covers about 3,200 square miles, and ranges in altitude from about 3,000 to 5,000 feet above sea level. The average annual precipitation in the area is about 16 inches and is sufficient to support grasses and some grains. Irrigation utilizing water diverted from the river and pumped from wells is extensively developed in the valleys of the South Platte River and its tributaries. The principal agricultural products are Corn, sugar beets, alfalfa, beans, wheat, barley, and livestock.

The rocks exposed in the area are sedimentary and range in age from Late Cretaceous to Recent. The Pierre shale underlies the entire area. The Fox Hills sandstone and the Laramie formation underlie the western part and the Chadron, Brule, and Ogallala formations underlie the eastern part. Pleistocene arid Recent alluvium underlies the valleys of the South Platte River and its tributaries. The Pierre shale ranges in thickness from about 2,500 feet near Paxton, Nebr., to about 6,500 feet near Hardin, Colo., and yields water in small quantities to wells in the vicinity of Sterling, Colo. Within the area, both the Fox Hills sandstone and the Laramie formation range in thickness from a featheredge to nearly 200 feet and yield small quantities of water to stock and domestic wells. Although a test hole near Proctor, Colo., was drilled 102 feet into the Chadron formation, the total thickness of the formation was not ascertained; no wells within the area covered by this investigation are known to derive water from the formation. The Brule formation ranges in thickness from a featheredge to more than 500 feet and yields water to wells from fractured or porous zones. The Ogallala formation ranges in thickness from a featheredge near Sedgwick, Colo., to about 350 feet near Paxton, Nebr., and yields large quantities of water to wells. The alluvium ranges in thickness from a featheredge at the edges of valleys to about 300 feet in some places in the valleys. The alluvium occurs in two physiographic forms Pleistocene and Recent terrace deposits and Recent floodplain deposits and yields abundant water to irrigation, public-supply, and other wells. Dune-sand deposits cover part of the area, range in thickness from a featheredge to about 100 feet, and yield water in small quantities to stock and domestic wells. Loess deposits cover much of the area and range in thickness from a featheredge to about 50 feet. Generally the loess is above the water table and is not known to yield water to wells.